Different road surfaces are characterized by different road surface friction coefficients (μ). For example, icy conditions may have a relatively low μ, while dry asphalt surfaces may have a relatively high μ. The road surface friction coefficient affects vehicle performance by influencing tire grip, for example.
In order to improve vehicle responsiveness to the road surface, vehicle systems attempt to estimate the road surface friction coefficient. One method of estimating the road surface friction coefficient is to calculate the self-aligning torque (SAT) of the vehicle's tires, and estimate μ based on the calculated self-aligning torque (SAT).
Conventional methods estimate a slope estimate for a linear region of self-aligning torque (SAT) as a function of slip angle (α). The slope estimate is then converted to an estimate of the road surface friction co-efficient using a look up table.
However, these methods provide inaccurate detection of low road surface friction coefficient surfaces at lower values of SAT, such as before SAT reaches a maximum value in the linear SAT region as a function of slip angle. The pre-saturation region of SAT versus slip angle corresponds to normal steering and moderate slip angle.
Furthermore, other methods of estimating the road surface friction coefficient based on SAT values rely on detecting the so-called “break point” of SAT, where a pneumatic trail collapses. The SAT break point is only encountered in high steering regions, i.e., in harsh driving maneuvers. These other methods therefore also suffer from the problem of not accurately predicting road surface friction coefficients at low SAT values.
Accordingly, it is desirable to provide improved methods and systems for determining road surface friction coefficients using self-aligning torque. It is further desirable to more accurately determine the road surface coefficients at lower values of self-aligning torque. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.